Toughening of an alumina-mullite composite by unbroken bridging elements

The toughness-curve (T-curve) behavior of a composite that is composed of 30 vol% spherical, polycrystalline, fine-grained alumina agglomerates dispersed throughout a constant-toughness 50-vol%-alumina-50-vol%-mullite matrix was compared to that for a coarse-grained agglomerate version of the composite, with the intent of identifying the operative toughening mechanisms in the latter. The T-curve behavior was evaluated using the indentation-strength method, and the underlying T-curves were deconvoluted from experimental data using an indentation-fracture-mechanics model. Compared to the T-curve of the coarse-grained composite, the toughness of the "fine-grained" composite increased from a similar initial toughness and over a similar crack-length range but to a lower saturation value. The T-curve of the fine-grained composite can be explained as being derived predominantly from unbroken bridging elements in the crack wake, set up by agglomerate-induced crack-path deflections in the matrix. This mechanism is proposed to function to the same degree and account for the majority of the observed toughening in the coarse-grained composite, with the remaining toughening increment being derived from intra-agglomerate grain bridging. Calculations suggest that unbroken bridging elements do not behave as simple elastic cantilevered beams that bend uniaxially with increasing crack-opening displacement.